1. Field of the Invention
The present invention relates to a double-tube type boring and kneading machine adapted to bore a foundation ground and to knead the soils and sands taken by the boring and mixed with a liquid such as a cement milk, and also relates to a method for improving a foundation ground by employing said double-tube type boring and kneading machine.
2. Prior Art
When conventionally improving a foundation ground (referred to as a weak ground hereinafter) composed of weak soil and sand strata, it is necessary to improve the weak ground together with a portion of an upper section of a tight solid earth or a rock (referred to as a solid ground) below the weak ground. The reason is because landslides often happen at a boundary between the weak ground and the solid ground in case that a stratum including them is inclined. Generally, when improving the weak ground, can be adopted such a method as kneading the soils and sands mixed with the binding liquid by means of a pressure of the liquid spouted from the leading end of of a rotary shaft while the binding liquid is being spouted to the soils and sands in that way. In that case, a sufficient stirring and mixing is not attained because only the pressure of the spouted liquid is utilized. Further, in another method, the boring is carried out while the binding liquid is discharged from the leading end of a rotary shaft, and at the same time the binding liquid and the soils and sands are mixed and kneaded by means of a stirring and kneading wing attached to the rotary shaft. In this case, however, it is necessary to rotate the rotary shaft at a high speed in order to sufficiently knead the soils and sands mixed with the binding liquid. In case that the rotary shaft is rotated at a high speed, the boring capability thereof is decreased and simulatneously it becomes impossible to bore a portion of the upper section of the solid ground. On one hand, in case that the rotary shaft is rotated at a low speed to increase the boring capability, the stirring and kneading becomes insufficient.
As a boring and digging machine capable of sufficiently stirring and kneading as well as keeping the boring capability at a high level, conventionally has been known the one disclosed in Japanese Patent Publication &lt;KOKOKU&gt; No. 01-29930. The double-tube type boring and kneading machine disclosed in Japanese Patent Publication &lt;KOKOKU&gt; No. 01-29930 has a double tubular shaft assembly comprising an outer tubular shaft and an inner shaft which both shafts are separately rotatable. The outer tubular shaft is adapted to rotate at a high speed and is provided only at its lower external surface with a stirring means. The inner shaft is adapted to rotate at a low speed and is provided at its leading end with a bit and a liquid discharge port.
In such a conventional double-tube type boring and kneading machine, since the dug soils and sands and the binding liquid are stirred and kneaded only by a stirring and kneading means attached to the outer tubular shaft, the stirring and kneading is insufficient.